Blue and Near-UV Phosphorescence from Iridium Complexes with Cyclometalated Pyrazolyl or N-Heterocyclic Carbene Ligands

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Abstract
Two approaches are reported to achieve efficient blue to near-UV emission from triscyclometalated iridium(III) materials related to the previously reported complex, fac-Ir(ppz)3 (ppz = 1-phenylpyrazolyl-N,C2‘). The first involves replacement of the phenyl group of the ppz ligand with a 9,9-dimethyl-2-fluorenyl group, i.e., fac-tris(1-[(9,9-dimethyl-2-fluorenyl)]pyrazolyl-N,C2‘)iridium(III), abbreviated as fac-Ir(flz)3. Crystallographic analysis reveals that both fac-Ir(flz)3 and fac-Ir(ppz)3 have a similar coordination environment around the Ir center. The absorption and emission spectra of fac-Ir(flz)3 are red shifted from those of fac-Ir(ppz)3. The fac-Ir(flz)3 complex gives blue photoluminescence (PL) with a high efficiency (λmax = 480 nm, φPL = 0.38) at room temperature. The lifetime and quantum efficiency were used to determine the radiative and nonradiative rates (1.0 × 104 and 2.0 × 104 s-1, respectively). The second approach utilizes N-heterocyclic carbene (NHC) ligands to form triscyclometalated Ir complexes. Complexes with two different NHC ligands, i.e., iridium tris(1-phenyl-3-methylimidazolin-2-ylidene-C,C2‘), abbreviated as Ir(pmi)3, and iridium tris(1-phenyl-3-methylbenzimidazolin-2-ylidene-C,C2‘), abbreviated as Ir(pmb)3, were both isolated as facial and meridianal isomers. Comparison of the crystallographic structures of the fac- and mer-isomers of Ir(pmb)3 with the corresponding Ir(ppz)3 isomers indicates that the imidazolyl-carbene ligand has a stronger trans influence than pyrazolyl and, thus, imparts a greater ligand field strength. Both fac-Ir(pmi)3 and fac-Ir(pmb)3 complexes display strong metal-to-ligand-charge-transfer absorption transitions in the UV (λ = 270−350 nm) and phosphoresce in the near-UV region (E0-0 = 380 nm) at room temperature with φPL values of 0.02 and 0.04, respectively. The radiative decay rates for fac-Ir(pmi)3 and fac-Ir(pmb)3 (5 × 104 s-1 and 18 × 104 s-1, respectively) are somewhat higher than that of fac-Ir(flz)3, but the nonradiative rates are two orders of magnitude faster (i.e., (2−4) × 106 s-1).